1. Field of the Invention
This invention relates to the field of implantable medical devices. Particularly, this invention is directed to coatings for drug delivery devices, such as drug eluting vascular stents.
2. Description of the State of the Art
In the treatment of vascular disorders, such as arteriosclerosis, intracoronary stents such as balloon expandable or self-expandable stents are now a standard adjunct to balloon angioplasty. Although a significant innovation for eliminating vasospasm, tacking intimal flaps to the vessel wall, and reduce negative remodeling, restenosis of the vessel following balloon angioplasty and stent replacement remains a persistent problem. Accordingly, stents are being modified to provide biological therapy for the treatment of restenosis.
Being made of a metallic material, such as stainless steel, bare stents have to be modified so as to provide an appropriate means for the deliver a drug. A polymeric coating on the surface of a stent is one method for providing a vehicle for the local delivery of a drug. A conventional coating used to achieve local drug delivery via stent can include a three-layer structure, as illustrated by FIG. 1. The three layer structure includes a drug-polymer layer 3 serving as a reservoir for the drug, a primer polymer layer 2 for improving the adhesion of the drug-polymer layer 3 to the surface of the stent 1, and a topcoat polymer layer 4 for reducing the rate of release of the drug from the drug polymer layer 3. The medicine to be administered will have a sustained release profile from drug-polymer layer 3 through the topcoat polymer layer 4.
For the effective treatment of restenosis, it is important to maintain the concentration of the drug at the treatment site at a therapeutically effective level for an acceptable period of time. Hence, controlling a rate of release of the drug from the stent is important, especially in such a way so as to provide a sustained, relatively constant, long-term release of the drug from the polymer matrix. In view of the foregoing, improved coatings and methods of fabricating the coatings for reducing the rate of release a therapeutic substance from stents are desired. The coatings should prolong the residence time of the drug in the patient, among other suitable characteristics, such as a steady rate of release.